Rapid Firecannon's passive has more effects and more damage than Kircheis Shard and therefore doesn't stack with it.

Strategy

This item has the least amount of attack speed out of all attack speed/crit chance items. Unlike Statikk Shiv, this item's energized strike is not applied in area of effect. In addition, it cannot crit, so it does less damage than Statikk Shiv.

Despite this, this item allows you to land a basic attack outside of your basic attack range. This is really good for situations where you need to take out a tower but enemies are defending it. You can also use it to poke your enemies from a safe distance, before engaging into a fight, which is especially useful if the enemies are strong enough to engage you if you enter into your basic attack range. Due to those reasons, Rapid Firecannon is one of the most commonly purchased attack speed/crit chance items.

This item is most commonly paired with Runaan's Hurricane, which make up for Rapid Firecannon's lack of waveclear compared to Statikk Shiv. It is also commonly purchased by caster ADCs which will purchase Essence Reaver instead of a second attack speed/crit chance item, as they do not really need a lot of attack speed and make good use of the extra range.

It is also commonly paired with Statikk Shiv. Although the damage from both items do not stack, this item extends the range which you can proc Statikk's AoE damage, in addition to energizing it faster and allowing it to be applied on turrets.